List of M-designations?

[StainlessSteelCynic]

I can't answer your entire question but I do know that the reason the Mark 19 is not given the M designation is because it was originally a USN weapons system and thus given a navy designation. Later the US Army adopted the weapon. If it had been the other way around, the USN would be using the grenade launcher as M*** rather than Mk19.

[sic1701]

Thanks for the Wikipedia link, Dog 6.

Cynic, the idea of the Army using something not preceeded by M-something-or-other kind of offends my sense of symmetry...like grunts would allow the squids to label something and make it stick!